This invention relates to a method of manufacturing electrical conductors having superconducting properties. More specifically, the invention relates to an improved method for manufacturing multifilament composite superconducting wire.
One present method for preparing multifilament composite superconducting wire requires that a cylindrical rod of ductile superconducting material be inserted into a tube of normal or nonsuperconducting metal to form a composite rod about 2 feet long. About 2000 of the composite rods, which are hexagonal in cross section to improve packing density, are tightly packed into an extrusion can of normal metal, which is then sealed to form an assembly, and reduced in cross section by various methods of cold and hot working which are well known in the art to produce a multifilament composite wire containing elements of the superconductor material in a matrix of ductile nonsuperconductor material.
The preparation of composite rods is difficult and time-consuming because of the close tolerances necessary to ensure a good bond between the metals after they have been worked. Each rod of superconducting material must be carefully straightened so that it can be inserted into the hexagonally shaped tube of normal metal. The close tolerances are also necessary to prevent the presence of any contamination. It is important that the hexagonal shape of the outer surface of the tubes be dimensionally accurate so that large numbers of composite rods can be tightly packed into the extrusion can, to prevent trapping of gas or other contaminants between the rods which would have a deleterious effect upon the superconductor. Thus preparation of composite superconducting wire by the aforedescribed method is time-consuming, expensive and due to the requirement for close tolerances, necessitates the acquisition of expensive components.
Some of the various uses for multifilament wire require that the wire have certain electrical characteristics which must be provided at the time of manufacture. For example, when the wire is to be used in certain magnets, it is preferred that the individual superconducting filaments in a multifilament wire be isolated from each other not only by the matrix of normal metal, but also by a layer of high resistance normally conducting material in order to reduce coupling and eddy currents. This is especially important for ac magnets that are to be used in some particle accelerators. At present this high resistance barrier is provided by applying a coating or layer of high resistance material such as cupronickel (Cu[10-30%]Ni) to the exterior of the hex-shaped tubes before they are packed into the extrusion can.
Conditions required for the production of certain types of superconducting wires such as high-temperature extrusion may cause the migration or diffusion of atoms of normal metal into the superconducting alloy and viceversa with a resulting destruction of the superconducting properties. This problem has been solved in the past by providing a diffusion barrier such as pure niobium on the surface of the rod of superconducting alloy before it is inserted into the tube of normal metal.